The present invention relates generally to rotary kilns and, more particularly, to a rotary kiln having an inner lining with a a sinusoidal wave-shaped face for pyro-processing material, such as cement, lime, lime sludge, and other minerals.
Conventional rotary kilns used for pyro-processing a wide variety of materials, such as cement, lime, lime sludge, rocks, and minerals, are commonly lined with bricks to protect the inner annular surface of the shell of the kiln. The bricks are of a refractory composition which is sufficiently resistant to the intense heat produced in the kiln to protect the shell from degradation. For example, the processing zone within the shell of a cement kiln attains a temperature of 2700.degree. F. and, in order to withstand such intense heat, the lining is preferably manufactured from an abrasive and heat resistant castable ceramic or brick material.
For kilns using a refractory lining of bricks, the tapered or specially-shaped bricks are secured to the inner annular surface of the shell so that the bricks extend along the inner circumference of the shell. The bricks form an annular ring about the processing zone and, in addition to protecting the steel shell, they reduce the heat loss through the steel shell to the external environment.
Moreover, the lining may be formed from a granular refractory material which is mixed with water to form a concrete-like material that is cast onto the inner annular surface of the shell. The particular configuration of the lining may be achieved by the use of forms and appropriate spacers which define the volume which is to be filled or cast with the refractory material. The lining may be formed by precasting an appropriate configuration which has a base shaped to conform to the inner annular surface of the shell. The base may be made of steel to facilitate attachment to the metal shell. After the refractory material is precast into the shape, it is inserted into the shell and secured to the inner annular surface by being bolted or welded thereto.
Conventional rotary kilns with present refractory lining designs are heat inefficient, resulting in a high fuel cost. For example, although the theoretical heat of formation of a Type I cement clinker is 420 kcal/kg, typical dry and wet process kilns consume far greater energy, approximately 1100 kcal/kg (38% heat efficiency) and 1300 kcal/kg (32% heat efficiency), respectively. Similarly, for lime kilns, typical heat efficiencies are in the range of about 40%. Such low heat efficiencies result from surplus heat being dissipated in the stack gases, lost by radiative heat transfer through the shell of the kiln and also being dissipated with the product itself.